.TH std::empty 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::empty \- std::empty

.SH Synopsis
   Defined in header <array>
   Defined in header <deque>
   Defined in header <flat_map>
   Defined in header <flat_set>
   Defined in header <forward_list>
   Defined in header <iterator>
   Defined in header <list>
   Defined in header <map>
   Defined in header <regex>
   Defined in header <set>
   Defined in header <span>
   Defined in header <string>
   Defined in header <string_view>
   Defined in header <unordered_map>
   Defined in header <unordered_set>
   Defined in header <vector>
   template< class C >                                          (since
   constexpr auto empty( const C& c ) ->                        C++17)
   decltype(c.empty());                                         (until
                                                                C++20)
   template< class C >                                          (since
   [[nodiscard]] constexpr auto empty( const C& c ) ->          C++20)
   decltype(c.empty());
   template< class T, std::size_t N >                                   (since
   constexpr bool empty( const T (&array)[N] )                          C++17)
   noexcept;                                                            (until
                                                        \fB(1)\fP             C++20)
   template< class T, std::size_t N >                                   (since
   [[nodiscard]] constexpr bool empty( const T                          C++20)
   (&array)[N] ) noexcept;                                  \fB(2)\fP
   template< class E >                                                          (since
   constexpr bool empty( std::initializer_list<E> il )                          C++17)
   noexcept;                                                                    (until
                                                                \fB(3)\fP             C++20)
   template< class E >                                                          (since
   [[nodiscard]] constexpr bool empty(                                          C++20)
   std::initializer_list<E> il ) noexcept;

   Returns whether the given range is empty.

   1) Returns c.empty().
   2) Returns false.
   3) Returns il.size() == 0.

.SH Parameters

   c     - a container or view with an empty member function
   array - an array of arbitrary type
   il    - an std::initializer_list

.SH Return value

   1) c.empty()
   2) false
   3) il.size() == 0

.SH Exceptions

   1) May throw implementation-defined exceptions.

.SH Notes

   The overload for std::initializer_list is necessary because it does not have a
   member function empty.

            Feature-test macro           Value    Std               Feature
   __cpp_lib_nonmember_container_access 201411L \fI(C++17)\fP std::size(), std::data(), and
                                                        std::empty()

.SH Possible implementation

.SH First version
   template<class C>
   [[nodiscard]] constexpr auto empty(const C& c) -> decltype(c.empty())
   {
       return c.empty();
   }
.SH Second version
   template<class T, std::size_t N>
   [[nodiscard]] constexpr bool empty(const T (&array)[N]) noexcept
   {
       return false;
   }
                                Third version
   template<class E>
   [[nodiscard]] constexpr bool empty(std::initializer_list<E> il) noexcept
   {
       return il.size() == 0;
   }

.SH Example


// Run this code

 #include <iostream>
 #include <vector>

 template<class T>
 void print(const T& container)
 {
     if (std::empty(container))
         std::cout << "Empty\\n";
     else
     {
         std::cout << "Elements:";
         for (const auto& element : container)
             std::cout << ' ' << element;
         std::cout << '\\n';
     }
 }

 int main()
 {
     std::vector<int> c = {1, 2, 3};
     print(c);
     c.clear();
     print(c);

     int array[] = {4, 5, 6};
     print(array);

     auto il = {7, 8, 9};
     print(il);
 }

.SH Output:

 Elements: 1 2 3
 Empty
 Elements: 4 5 6
 Elements: 7 8 9

.SH See also

   ranges::empty checks whether a range is empty
   (C++20)       (customization point object)
